1. Field of the Invention
This invention relates to a method and apparatus for measuring bone mineral density and may be used for osteoporosis screening. More specifically, the invention relates to a method and apparatus for measuring bone mineral density using electromagnetic wave radiation and storage layer radiation screens.
2. Description of Related Art
Osteoporosis is a bone disease in which bones become thinner and more porous, commonly resulting in fractures of the bones. More than 1.5 million osteoporosis-related fractures occur each year in the United States, usually in the spine, hip and wrist. Osteoporosis is most common in older women and about 25% of women older than 60 years have osteoporosis.
Bone density testing, which measures bone mineral content, is used to diagnose osteoporosis. A low bone density may indicate a risk for fractures in the future. The test can also be used to determine a rate of bone mineral loss in those not receiving treatment, and a rate of bone gain in those being treated.
Several different methods and apparatuses have been developed for measuring bone mineral density. These different methods and apparatuses can be broadly divided based on whether they are single or dual energy X-ray systems.
Mineral loss in a person""s bones can be estimated from a single X-ray image of a body part. One potential difficulty associated with single X-ray images is that it is difficult to determine how much of the X-ray image is due to hard tissue (e.g., bones) and how much is due to soft tissue (skin, muscle, ligaments, etc.).
Dual-energy x-ray absorptometry (DXA) is a technique which has been developed which uses two x-ray images obtained using x-rays of different energy levels to compensate for the fact that hard tissue is surrounded by soft tissue that also contributes to an x-ray image. Dual energy systems use two images to obtain a set of two simultaneous equations for each pixel in the images and then solve those equations to determine the amount of x-rays that was absorbed by the bone. Shimura (U.S. Pat. No. 5,187,731) describes a method for quantifying bone mineral using a dual energy radiation system.
The two x-ray images are obtained using x-rays with different energy levels to compensate for tissue variations in quantifying bone mass in an x-ray image. Typically, existing DXA systems rely on known x-ray absorption characteristics of hard tissue and soft tissue to both high-energy and low-energy radiation. Some systems use an x-ray image of a wedge of material with bone-like x-ray attenuation properties (e.g., aluminum) to calibrate the system, where the thickness and density of each part of the wedge is known in advance.
Existing DXA measurements generally require expensive equipment that is usually available only in specialized facilities. This equipment is typically complex, and the test results must be interpreted by a skilled person (e.g., a radiographer), resulting in significant cost of labor. In addition, because the test results must be interpreted by humans, existing tests are not highly repeatable. It would be advantageous to remove human variations and determine more accurately the progression of osteoporotic condition over a period of time, such as one year intervals, to determine the progression of bone loss. The need for the interpretation of test results also makes it more difficult to return these results to the patient instantly.
U.S. Pat. Nos. 5,150,394 and 5,465,284 describe systems which measure bone density using x-ray radiation at two different energy levels. In these patents, x-ray radiation of two intensity levels is transmitted through a portion of the patients body to a scintillator which converts the x-rays into visible light. The visible light emitted by the scintillator is provided to a charge-coupled device (CCD), which in turn converts the visible light into an electrical signal. The system then forms an image of the body from the electrical signal, and determines the density the patient""s bone from the image.
U.S. Pat. Nos. 5,852,647 and 5,898,753 also describe dual energy level systems. In U.S. Pat. No. 5,898,753, a system is described which uses CMOS wafers instead of CCD wafers which allows larger sensors to be manufactured more readily.
The present invention relates to apparatuses and methods for forming and reading a radiation image of an object.
In one embodiment, an apparatus is provided for forming and reading a radiation image of an object which comprises: a platform adjacent to which may be placed an object of which a radiation image is to be taken; a cylindrically shaped rotatable drum; a storage layer radiation screen mounted adjacent a surface of the rotatable drum; an electromagnetic wave radiation source positioned relative to the storage layer radiation screen and the platform such that radiation from the electromagnetic wave radiation source which traverses the platform and the object adjacent the platform is absorbed by the storage layer radiation screen; an image acquisition optical system positioned adjacent the drum, the image acquisition optical system including an excitation system for directing an excitation beam in the direction of the drum, the excitation beam causing energy to be emitted from portions of the screen which are contacted with the excitation beam as the drum is rotated, and an emission collecting system for collecting the energy emitted from the screen; an optics driver for moving the image acquisition optical system in a direction parallel to the rotational axis of the drum as the drum is rotated; and a drum drive mechanism which causes the rotatable drum to rotate. The system may further include embedded software for controlling system functions. Functions which the software may perform include, but are not limited to: interfacing with the system operator in accepting patient data, providing system status information, automatic image processing and analysis, and presentation of exam results.
In one variation, the apparatus may further include a screen erasing mechanism for erasing the storage layer radiation screen. According to this variation, the apparatus is able to be reused without having to remove the screen between scans. The screen erasing mechanism may be positioned adjacent the rotatable drum and used to release any radiation energy remaining stored on the screen after the screen has been read.
The screen erasing mechanism may also be positioned within the image acquisition optical system and moved by a same mechanism as the image acquisition optical system. The screen erasing mechanism may be designed to erase the screen as the screen is rotated without having to move the screen erasing mechanism. The screen erasing mechanism may be designed to provide energy over a sufficiently large area that the screen may be erased without either the erasing mechanism or the screen being moved.
In another embodiment, an apparatus is provided for forming and reading a radiation image of an object which comprises: a platform adjacent to which may be placed an object of which a radiation image is to be taken; a rotatable platter; a storage layer radiation screen mounted adjacent a surface of the rotatable platter; an electromagnetic wave radiation source positioned relative to the storage layer radiation screen and the platform such that radiation from the electromagnetic wave radiation source which traverses the platform and the object adjacent the platform is absorbed by the storage layer radiation screen; an image acquisition optical system positioned adjacent the platter, the image acquisition optical system including an excitation system for directing an excitation beam in the direction of the drum, the excitation beam causing energy to be emitted from portions of the screen which are contacted with the excitation beam as the platter is rotated, and an emission collecting system for collecting the energy emitted from the screens; an optics driver for moving the image acquisition optical system in a direction radially relative to the rotational axis of the platter as the platter is rotated; and a platter drive mechanism which causes the rotatable platter to rotate.
In one variation of this embodiment, the apparatus may further include a screen erasing mechanism for erasing the storage layer radiation screen. According to this variation, the apparatus is able to be reused without having to remove the screen between scans. The screen erasing mechanism may be positioned adjacent the rotatable platter and used to release any radiation energy remaining stored on the screen after the screen has been read. The screen erasing mechanism may optionally be contained within a screen erasing module which moves in a direction along the radius of the rotatable platter simultaneously with the rotation of the platter. The screen erasing mechanism may also be positioned within the image acquisition optical system and moved by a same mechanism as the image acquisition optical system. The screen erasing mechanism may be designed to erase the screen as the screen is rotated without having to move the screen erasing mechanism. The screen erasing mechanism may be designed to provide energy over a sufficiently large area that the screen may be erased without either the erasing mechanism or the screen being moved.
In another embodiment, an apparatus is provided which comprises: a storage layer radiation screen; an electromagnetic wave radiation source positioned relative to the storage layer radiation screen such that radiation from the electromagnetic wave radiation source which traverses an object adjacent the storage layer radiation screen is absorbed by the storage layer radiation screen; an image acquisition optical system including an excitation system for directing an excitation beam to the screen to cause energy to be emitted from portions of the screen which are contacted with the excitation beam, and an emission collecting system for collecting the energy emitted from the screen; and computer executable logic which is capable of taking data corresponding to energy emitted from the screen and signals corresponding to a reference image, normalizing the data corresponding to energy emitted from the screen relative to the reference image, and computing bone mineral density based on the normalized data.
According to this embodiment, data corresponding to energy emitted from the screen may be normalized relative to the reference image pixel by pixel. Also according to this embodiment, the storage layer radiation screen may have a non-planar surface, the computer executable logic further comprising logic for correcting for geometric distortion arising from the screen having a non-planar surface. The computer executable logic may compute bone mineral density based on a ratio between absorption in a bone region and absorption in a soft tissue region. The computer executable logic may also compute bone mineral density based on the equation   BMD  =            1      n        ⁢    Δ    ⁢          xe2x80x83        ⁢    x    ⁢          xe2x80x83        ⁢    Δ    ⁢          xe2x80x83        ⁢    y    ⁢                  ∑                  i          =          1                n            ⁢              xe2x80x83            ⁢              ln        ⁢                              I            wi                                I                                          (                                  b                  +                  w                                )                            ⁢              i                                          
where
Iwi is absorption in the soft tissue region of the image;
I(b+w)i is absorption in the bone region which includes soft tissue.
In another embodiment, the apparatus comprises: a storage layer radiation screen having a non-planar surface; an electromagnetic wave radiation source positioned relative to the storage layer radiation screen such that radiation from the electromagnetic wave radiation source which traverses an object adjacent the storage layer radiation screen is absorbed by the storage layer radiation screen; an image acquisition optical system including an excitation system for directing an excitation beam to the screen to cause energy to be emitted from portions of the screen which are contacted with the excitation beam, and an emission collecting system for collecting the energy emitted from the screen; and computer executable logic which is capable of taking data corresponding to energy emitted from the screen and signals corresponding to a reference image, correcting for geometric distortion arising from the screen having a non-planar surface, and computing bone mineral density based on the corrected data. According to this embodiment, the storage layer radiation screen may be positioned on a rotatable cylindrically shaped rotatable drum.
The apparatuses of the present invention may be designed to operate as a single energy system and/or as a dual energy system. When designed to operate as a dual energy system, the electromagnetic wave radiation source is capable of emitting photons at two different energy levels. Alternatively, the apparatus may include a mechanism for altering an amount of energy delivered to the storage layer radiation screen.
According to any of the above apparatus embodiments, an object plate may be positioned adjacent the platform. The object plate may comprise an optically opaque but electromagnetic wave energy transmissive material. The object plate may include guides for the middle three fingers of a patient""s hand.
The object plate may optionally be removable from the platform.
The object plate may optionally further include a hard tissue reference, such as a linear wedge of a material with energy absorption characteristics similar to those of human bone. A preferred material for the wedge is aluminum.
Several methods are also provided for forming and reading a radiation image of an object. In one embodiment, the method comprises: placing an object of which a radiation image is to be taken adjacent a platform that is positioned between a storage layer radiation screen and an electromagnetic wave radiation source such that radiation from the electromagnetic wave radiation source which traverses the object is absorbed by the storage layer radiation screen; forming a latent radiation image of the object on the storage layer radiation screen by causing the electromagnetic wave radiation source to emit radiation, a portion of the emitted radiation traversing the object and being absorbed by the storage layer radiation screen; and reading the latent radiation image of the object from the storage layer radiation screen.
According to one variation of the method, the storage layer radiation screen is mounted on a cylindrically shaped rotatable drum, reading the latent radiation image of the object from the storage layer radiation screen including causing energy to be emitted from portions of the screen which are contacted with an excitation beam as the drum is rotated, and collecting the energy emitted from the screen.
When the screen is positioned on the cylindrically shaped rotatable drum, it is noted that the image formed on the drum is distorted due to the fact that the screen is curved adjacent a curved surface of the drum. The method may further include correcting for distortion in the read latent radiation image due to the curvature of the screen adjacent the curved surface of the drum.
According to another variation of the method, the storage layer radiation screen is mounted on a rotatable platter, reading the latent radiation image of the object from the storage layer radiation screen including causing energy to be emitted from portions of the screen which are contacted with an excitation beam as the rotatable platter is rotated, and collecting the energy emitted from the screen. According to this variation, reading the latent radiation image may include moving an image acquisition optical system radially as the rotatable platter is rotated.
In another embodiment, the method comprises: placing an object of which a radiation image is to be taken between a storage layer radiation screen and an electromagnetic wave radiation source such that radiation from the electromagnetic wave radiation source which traverses the object is absorbed by the storage layer radiation screen; forming a latent radiation image of the object on the storage layer radiation screen by causing the electromagnetic wave radiation source to emit radiation, a portion of the emitted radiation traversing the object and being absorbed by the storage layer radiation screen; reading the latent radiation image of the object from the storage layer radiation screen; and normalizing the latent radiation image using a reference image. Normalizing the latent radiation image may include normalizing each pixel of the latent radiation image relative to a corresponding pixel on the reference image. The method may further include calculating bone mineral density from the normalized latent radiation image. Computing bone mineral density may be based on a ratio between absorption in a bone region and absorption in a soft tissue region and may be based on the equation   BMD  =            1      n        ⁢    Δ    ⁢          xe2x80x83        ⁢    x    ⁢          xe2x80x83        ⁢    Δ    ⁢          xe2x80x83        ⁢    y    ⁢                  ∑                  i          =          1                n            ⁢              xe2x80x83            ⁢              ln        ⁢                              I            wi                                I                                          (                                  b                  +                  w                                )                            ⁢              i                                          
where
Iwi is absorption in the soft tissue region of the image;
I(b+w)i is absorption in the bone region which includes soft tissue.
In yet another embodiment, a method for forming and reading a radiation image of an object, the method comprising: placing an object of which a radiation image is to be taken between a storage layer radiation screen having a non-planar surface and an electromagnetic wave radiation source such that radiation from the electromagnetic wave radiation source which traverses the object is absorbed by the storage layer radiation screen; forming a latent radiation image of the object on the storage layer radiation screen by causing the electromagnetic wave radiation source to emit radiation, a portion of the emitted radiation traversing the object and being absorbed by the storage layer radiation screen; reading the latent radiation image of the object from the storage layer radiation screen; and correcting for geometric distortion arising from the screen having a non-planar surface. According to the method, normalizing the latent radiation image may include normalizing each pixel of the latent radiation image relative to a corresponding pixel on the reference image. The method may further include calculating bone mineral density from the normalized latent radiation image.
According to the above method embodiments, the storage layer radiation screen is preferably stationary relative during the formation of the latent radiation image.
According to the above apparatuses and methods, the object may be any part of the body for which bone mineral density is to be measured. Examples of body parts that may be read include, but are not limited to a part of an arm or leg, preferably a part of a hand or foot, more preferably a wrist, fingers and/or toes. In one particular embodiment, the part of the body includes the middle three fingers of a person""s hand.
In regard to each of the above embodiments, the platform may include a hard tissue reference, such as a linear wedge of a material with electromagnetic wave radiation absorption characteristics similar to those of human bone. In a preferred embodiment, the hard tissue reference is made of aluminum. The hard tissue reference preferably has a known thickness profile which may be used to calibrate the system.
It is noted that while a reference object may be included in or used with the system for the purpose of a diagnostic of the x-ray source, the system of the present invention are designed such that the reference object is not required.
In a preferred embodiment, a plurality of different systems are tested using a set of known test objects in order to calibrate the gain and offset for each of the plurality of different systems so that inter-system variability is reduced. By doing inter-system calibration, the need for a reference object is reduced.
Because of variations in x-ray intensity across the x-ray exposure field, and/or the presence of fixed defects in the storage layer radiation screen, the methods of the present invention may further include using a reference image taken when no external object is present on the hand support plate and using the reference image to correct the read latent radiation image. The reference image serves to normalize each pixel of the storage radiation screen so that different readings of latent radiation images may be accurately compared. In a preferred embodiment, the reference image is stored in memory and used with a series of read latent radiation images. Alternatively, a reference image may be taken for each read latent radiation image.